Power conversion systems play a significant role in converting energy produced by alternate energy sources, such as photovoltaic (PV) arrays and wind converters, into an optimized power form for supply to the electric grid. Insulated-gate bipolar transistor (IGBT) inverters or metal-oxide-semiconductor field-effect transistor (MOSFET) devices are often fundamental components in any power conversion system. In these power conversion systems, IGBT/MOSFET gate control is critical to optimizing output power.
Maximizing efficiency is an important consideration when optimizing the conversion of power from one form to another.
As is well known in the art, the peak semiconductor temperature is a function of the real and reactive power components. Because of IGBT temperature limitations, among other factors, conventional power conversion systems attempt to optimize only real power. By way of background, and as illustrated in FIG. 1, the real power and reactive power outputs of a power converter can be specified by a PQ curve.
In FIG. 1, a PQ curve 100 represents the interplay between real power and reactive power, for a fixed value of peak semiconductor temperature in a 1 MW inverter. In FIG. 1, for example, the x and y axes represent reactive power (Volt-Amp Reactive, VAR) and real power (watts, W), respectively. The curve 100 depicts changes in the power factor (PF) at a load. For example, changes in PF (e.g., as a function of amperage and voltage) can be measured in terms of power in kilowatts. In the exemplary curve 100, as PF changes in terms of reactive power instead of real power, IGBT temperatures typically get hotter.
In the curve 100, the total reactive power is at a minimum when the total real power is at its peak. Conventional power conversion systems are mainly concerned with real power. For example, in a PV array attempting to put 1 megawatt (MW) on the electric grid, conventional IGBT controls would be optimized to provide this 1 megawatt under optimal temperature conditions. That is, conventional systems simply focus on optimizing real power at the expense of reactive power. In some scenarios, however, there is also a need to focus on reactive power.
For example, in providing power to the electric grid if the line voltage is low or high, reactive power can be used to adjust the line voltage to reduce these high/low fluctuations. Thus, in many scenarios, optimizing reactive power can have significant benefits. In conventional systems, however, maximizing reactive power should not require sacrificing real power.
The particular shape of the PQ curve 100 depends on several factors, such as how filters are implemented, as well as other components. For example, the PQ curve 100 is slightly offset to the right. To get more reactive power or VARs to the right side than on the left, factors such as those above can be adjusted to change the shape of the PQ curve 100. The goal, however, is to be able to get as much reactive power as real power, or even more.
Factors limiting optimization of reactive power include IGBT switching and conduction losses. Reducing these losses is necessary for optimizing the PF of the IGBT conversion system and maximizing reactive power.